Fuel injection system

ABSTRACT

A fuel injection system for an internal combustion engine includes electromagnetic injection valves controlled by a fuel control unit which receives signals from a camshaft actuated switch, a position-dependent throttle transducer and an oxygen sensor. When the oxygen sensor changes output levels, the transmission of this information is delayed, by the action of a switching transistor controlled by a monostable multivibrator, for a period of time equal to the internal time constant of the multivibrator.

BACKGROUND OF THE INVENTION

The invention relates to a fuel injection system with an integralcontroller for changing the fuel-air mixture supplied to the internalcombustion engine under the control of an oxygen sensor located in theexhaust gas path of the internal combustion engine (lambda control).

It is known to change the mass ratio of components in the fuel-airmixture supplied to an internal combustion engine in dependence onexhaust gas composition, with the aid of an oxygen sensor located in theexhaust gas stream of the internal combustion engine. The known systemusually includes a controller which produces an appropriate increase ordecrease of the instanteous fuel quantity added to the combustion airstream and it does so in dependence on the output signal of the oxygensensor. It is known that this type of adjustment of the mass ratio ofthe fuel-air mixture may be employed in internal combustion enginesusing carburetors as well as in those using fuel injection systems. Thecontrollers which change the mass ratio of the fuel-air mixture normallyand preferably have integral control behavior so that, when theeffective value deviates for an extended period of time, the effortapplied to correct the mass ratio increases continually. In a known fuelinjection system of this type, the only control effort is that whichaims at an air number of λ=1.0. Due to the fact that the characteristicoutput voltage of the oxygen sensor changes with finite slope, it ispossible to obtain a very small change in the air number by varying thetriggering level of the subsequent threshold switch, but this permits achange of the air number only in the direction of a richer fuel-airmixture and does not provide the desired range or variation which shouldbe approximately ±5% about the value λ=1.0 so that the internalcombustion engine might be operated in an arbitrarily selectable regionwhich lies between λ=0.95 up to λ=1.05 while retaining all theadvantages of so-called λ-control.

OBJECT AND SUMMARY OF THE INVENTION

It is a principal object of the invention to provide a fuel injectionsystem of the known and above described type which is capable ofregulating the fuel-air mixture supplied to the internal combustionengine in a region in which the air number λ varies betweenapproximately 0.95 up to approximately 1.05, in dependence on thecomposition of the exhaust gas of the engine.

This object is attained, according to the invention, by providing acontrol system which includes a comparator circuit that receives thecontrol voltage from the oxygen sensor. The output of the comparator isconnected to the input of an integrating circuit and to an electronicswitch that includes a monostable multivibrator. After the occurrence ofa voltage jump at the output of the oxygen sensor, the change over ofthe direction of integration of the integrator is delayed for anadjustable period of time, equal to the time constant of the monostablemultivibrator. The integrator is coupled to an electronic fuelcontroller system which, in turn, actuates the final control elementwhich changes the fuel-air mixture composition.

The invention provides that, when the output voltage of the comparatorchanges to the positive level, the input to the integrator may beshort-circuited during the time the monostable multivibrator resides inits unstable state.

The invention further provides that the inputs of the integrator may beheld at a high potential by means of a transistor whenever the voltageat the output of the comparator changes to the negative level.

Another feature of the invention is that the time constant of themonostable multivibrator can be changed in dependence on the aspiratedair quantity and/or the rpm.

Another embodiment of the invention provides that when the outputvoltage from the comparator changes in either sense, the inputs of theintegrator may be disconnected during a period equal to the timeconstant of the monostable multivibrator.

Thus, the invention provides an apparatus including electronic circuitrywhich permits a fuel injection system to operate the internal combustionengine with a fuel mixture which can be made either richer or leanerthan a stoichiometric mixture.

The invention will be better understood as well as further objects andadvantages will become more apparent from the ensuing detailedspecification taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an overall diagram of an internal combustion engine and anassociated fuel injection system according to the invention, including ablock diagram of the electronic fuel control circuit.

FIG. 2 is a block diagram of a fuel control circuit similar to thatshown in FIG. 1, but for the condition λ 1.0.

FIG. 3 is a circuit diagram showing the internal detail of the circuitassociated with the monostable multivibrator in FIGS. 1 and 2.

FIG. 4 is a circuit diagram of a second embodiment of the circuitassociated with the monostable multivibrator in FIGS. 1 and 2; and

FIG. 5 is a diagram showing the output voltage of the comparator circuitas a function of time in the different embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fuel injection system depicted in FIG. 1 is intended to service afour-cylinder four-cycle internal combustion engine 10. This fuelinjection system includes, as essential constituents, fourelectromagnetically actuatable fuel injection valves 11, which aresupplied with fuel from a distributor 12 through individual fuel lines13. The system further includes an electronically driven fuel supplypump 15, a pressure regulator 16 which controls the fuel pressure to apredetermined constant value, and it also includes an electric controland regulating mechanism which will be described in detail below. Thismechanism is triggered twice during each camshaft revolution by means ofa signal generator 18 operatively coupled to the camshaft 17 and thusdelivers a rectangular electrical pulse S which is used to control theinjection valves 11. The pulse width t_(i) shown in the drawingdetermines the opening time of the injection valves and thus alsodetermines the quantity of fuel which is delivered by the injectionvalves during their opening time, due to an internal constant pressureof approximately 2 bar. Each of the magnetic windings 19 of theinjection valves is connected to an individual decoupling resistor 20.All the resistors 20, are in turn, connected to a common amplifying andpower stage belonging to an electronic control unit 21 which includes atleast one power transistor whose emitter-collector path is connected inseries with the decoupling resistors 20 and, hence, with the magneticwindings 19, whose other end is grounded.

In the operation of mixture-compressing and externally ignited internalcombustion engines of this type, the fuel quantity provided to aparticular cylinder during each piston suction stroke is so chosen thatit may be completely combusted during the subsequent power stroke. Highengine efficiency requires that no substantial amounts of unused airremain in the cylinder after the power stroke. To provide the desiredstoichiometric ratio of aspirated air and fuel, the induction tube 25 ofthe internal combustion engine includes an air flow rate meter LM,located downstream of a filter 26 but upstream of a throttle butterflyvalve 28 which may be adjusted by a gas pedal 27. The flow rate meter 11consists substantially of a baffle plate 30 and a variable potentiometerR whose adjustable tap 31 is moved by the baffle plate. The electricaloutput of the air flow rate meter LM is fed to the electronic controlunit 21 whose own output supplies the injection pulses of width t_(i).

The electronic control unit 21 includes two transistors in push-pulloperation and connected in mutual feedback configuration and it alsoincludes an energy storage device which may be a capacitor or aninductor. The duration of the discharging process of the energy storagedevice determines the opening duration t_(i) for the injection valves.Prior to each discharging process, the energy storage must be charged inan appropriate manner.

The charging process of the energy storage device is accomplished by aswitch, embodied in this example by a signal generator 18 which isactuated in synchronism with the crankshaft rotation, which providesthat the energy storage device is connected to a source of electriccharge during a predetermined, constant angular motion of thecrankshaft. The switch 18 thus provides a charging pulse LI which makesavailable a charging current during this time. At the same time, thefuel controller 21 receives information regarding the air quantityadmitted through the induction tube of the engine during this interval.In the present case, let it be assumed that the signal generator 18,which may also be embodied in a practical situation by a bi-stablemultivibrator clocked by ignition pulses, is closed over a crankshaftangle of 180° and is then opened over the remaining angle of 180°.

In that case, it is possible to achieve as nearly exact a regulation ofthe air number λ as desired by providing an oxygen sensor 34 in theexhaust pipe 33 of the internal combustion engine 10. This sensor 34delivers a control voltage in dependence on the exhaust gas compositionand this voltage is fed to the input of the comparator 35 whose outputis connected through a diode 36 to the input of an integrator 37 and isalso connected to the input of a monostable multivibrator 38. The anodeof the diode 36 is connected through a resistor 39 to the positivevoltage supply line and its cathode is connected to the output of thecomparator 35. The output of the monostable multivibrator 38 isconnected to the base of a transistor 40, whose emitter is grounded andwhose collector is connected to the input of the integrator 37. Theoutput voltage of the integrator is fed to the electronic fuelcontroller 21 which forms the injection pulses t_(i).

It is a principal object of this invention to provide an apparatus thatcan operate the engine at an air number λ which is different from unity(1) while using an oxygen sensor which changes voltage exactly at λ=1.For this purpose, it is provided that the reversal of the direction ofthe change of the output voltage from the integrator 37, which wouldnormally begin when the output voltage from the oxygen sensor switchesabruptly, is delayed for a predetermined time t_(v). The circuit shownin FIG. 1 makes possible a control of the fuel-air mixture so that theair number λ is greater than 1. This purpose is achieved by providingthat, when the output of the comparator 35 switches to a positive value,the monostable multivibrator 38 is triggered and its output renders thetransistor 40 conducting for a time t_(v), corresponding to the timeconstant of the monostable multivibrator 38, and thus holds the input ofthe integrator 37 at zero potential. During this time t_(v), theintegrator 37 continues to exert control in the direction of a leanermixture even though the oxygen sensor 34 has already signalled that aricher mixture is required.

FIG. 2 is an electronic control circuit which is very similar to thatshown in FIG. 1 but which permits control of the air number λ to valueswhere λ is less than 1 (rich mixture). For this purpose, the diode 42 isreversed from the position in FIG. 1, so that its anode is now connectedto the output of the comparator 35, while the output of the monostablemultivibrator 38 is connected to the base of a transistor 41 whoseemitter is connected to the positive supply line and whose collector iscoupled to the input of the integrator 37. Engine control with a richermixture than a stoichiometric mixture is achieved by holding the inputof the integrator 37 at a high potential for a predetermined time t_(v)after the output voltage from the comparator 35 shifts in the negativedirection. Thus the integrator continues to provide a control signal fora richer mixture even though the oxygen sensor 34 has already signalledthat a leaner mixture is required. FIG. 5a is a diagram showing theoutput voltage from the integrator 37 as a function of time during acontrol process wherein λ is less than 1.

FIG. 5b is a diagram of the integrator voltage as a function of timewhen λ is greater than 1.

Since the dead time and the general response characteristics of thecontroller are strongly dependent on the air throughput of the internalcombustion engine, it is useful to be able to change the delay timet_(v) as a function of the air throughput and/or the rpm of the engine.FIG. 3 is a circuit associated with a monostable multivibrator 38 whichpermits a throughput-dependent control process. A current source, formedby a transistor 46 and a series resistor 47 varies the current used forcharging the timing capacitor 45. An RC member, formed by a resistor 49and a capacitor 48 is used to generate a throughput dependent DCpotential from the injection pulse t_(i), which is fed to the base oftransistor 46 and controls the current I in the desired manner.

FIG. 4 shows a monostable multivibrator 38 whose time constant t_(v) ischanged by applying a voltage at point A; this voltage is also derivedfrom the injection pulses t_(i), in the manner shown in FIG. 3.

Yet another possibility to delay the reversal of the direction ofintegration of the integrator 37, consists in holding the output voltagefrom the integrator constant for a predetermined period of time t_(v)when a voltage shift occurs at the output of the comparator 35. This maybe done by disconnecting the integrator input, for example through aswitching transistor controlled by the multivibrator 38. The timebehavior of the integrator output voltage for this case is shown in FIG.5c.

Construction of electronic control unit 21 is known, for example, forU.S. Pat. No. 3,750,631.

What is claimed is: .[.1. A fuel-air control system for an internalcombustion engine, including a controller with integral controlcharacteristic for controlling the fuel-air mixture admitted to theengine (λ-control process) and including predetermined time period..Iaddend. .Iadd.
 8. A fuel-air control system for an internal combustionengine, including a controller with integral control characteristic forcontrolling the fuel-air mixture admitted to the engine (λ-controlprocess) and including(A) an oxygen sensor, located in the exhaustsystem of the engine, and capable of providing an output signal; theimprovement in said fuel-air system comprising: (B) a comparatorcircuit, for comparing said output signal from said oxygen sensor with areference value and providing an output signal; (C) an integratingcircuit, whose sole control signal input is connected to the output fromsaid comparator circuit and which delivers a changeable output signal;(D) a timing circuit connected to the output from said comparatorcircuit and which delivers an output signal that in addition to theoutput of the comparator circuit is continuously connected to the inputof said integrating circuit, the output signal from said timing circuitinhibits the transmission of said output signal from said comparator tosaid integrating circuit for delaying the change of its output signal inone direction for a predetermined period of time thereby delaying in onedirection the response of said integrating circuit to changes in theoutput signal from said oxygen sensor; (E) a control unit, connected tothe output from said integrating circuit, for providing injection valvecontrol signals; (F) an air-fuel rate adjusting device connected to saidcontrol unit; and (G) signal generator means, connected to said enginefor delivering an rpm-dependent output signal which is connected to saidtiming circuit to determine the duration of said predetermined period oftime. .Iaddend.